CSC139Operating Systems

Lecture 22

Networking II

Adapted from Prof. John Kubiatowicz's

lecture notes for CS162

http://inst.eecs.berkeley.edu/~cs162

Copyright © 2006 UCB

Lec 22.2

Review: Hierarchical Networking (The Internet)

• How can we build a network with millions of hosts?

– Hierarchy! Not every host connected to every other one

– Use a network of Routers to connect subnets together

subnet1

subnet2

Router

Othersubnets

Router

Router

TranscontinentalLink

subnet3Othersubnets

Lec 22.3

Review: Network Protocols• Protocol: Agreement between two parties as to

how information is to be transmitted– Example: system calls are the protocol between

the operating system and application– Networking examples: many levels

» Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented)

» Link level: packet formats/error control (for instance, the CSMA/CD protocol)

» Network level: network routing, addressing» Transport Level: reliable message delivery

• Protocols on today’s Internet:

Ethernet ATM Packet radio

IP

UDP TCP

RPCNFS WWW e-mail ssh

Physical/Link

Network

Transport

Lec 22.4

Review: Network Layering• Layering: building complex

services from simpler ones– Each layer provides services

needed by higher layers by utilizing services provided by lower layers

• Our goal in the following is to show how to construct a secure, ordered, arbitrary-sized message service routed to anywhere:

SecureInsecure

SynchronousAsynchronous

Routed anywhereOnly on local area net

Process-to-processMachine-to-machine

ReliableUnreliable

OrderedUnordered (sometimes)

Arbitrary SizeLimited Size

Abstraction: MessagesPhysical Reality: Packets

Lec 22.5

Review: Basic Networking Limitations• The physical/link layer is pretty limited

– Packets of limited size » Maximum Transfer Unit (MTU): often 200-1500

bytes – Packets can get lost or garbled– Hardware routing limited to physical link or

switch– Physical routers crash/links get damaged

» Baltimore tunnel fire (July 2001): cut major Internet links

• Handling Arbitrary Sized Messages:– Must deal with limited physical packet size – Split big message into smaller ones (called

fragments)» Must be reassembled at destination» May happen on demand if packet routed through

areas of reduced MTU (e.g. TCP)– Checksum computed on each fragment or whole

message• Datagram: an independent, self-contained

network message whose arrival, arrival time, and content are not guaranteed

• Need resilient routing algorithms to send messages on wide area

– Multi-hop routing mechanisms– Redundant links/Ability to route around failed

links

Lec 22.6

Review: Performance Considerations• Some performance metrics

– Overhead: CPU time to put packet on wire– Throughput: Maximum number of bytes per

second» Depends on “wire speed”, but also limited by

slowest router (routing delay) or by congestion at routers

– Latency: time until first bit of packet arrives at receiver

» Raw transfer time + overhead at each routing hop

• Contributions to Latency– Wire latency: depends on speed of light in wire or

fiber» about 1–1.5 ns/foot

– Router latency: depends on internals of router» Could be < 1 ms (for a good router)» Question: can router handle full wire throughput?

Router Router

LR1 LR2LW1 LW2 Lw3

Lec 22.7

Goals for Today

• Networking– Reliable Messaging

» TCP windowing and congestion avoidance

– Two-phase commit

Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne

Note: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated from my lecture notes by Kubiatowicz.

Lec 22.8

Sample Computations• E.g.: Ethernet within Soda

– Latency: speed of light in wire is 1.5ns/foot, which implies latency in building < 1 μs (if no routers in path)

– Throughput: 10-1000Mb/s– Throughput delay: packet doesn’t arrive until all

bits » So: 4KB/100Mb/s = 0.3 milliseconds (same order as

disk!)• E.g.: ATM within Soda

– Latency (same as above, assuming no routing)– Throughput: 155Mb/s– Throughput delay: 4KB/155Mb/s = 200μ

• E.g.: ATM cross-country – Latency (assuming no routing):

» 3000miles * 5000ft/mile 15 milliseconds– How many bits could be in transit at same time?

» 15ms * 155Mb/s = 290KB– In fact, BerkeleyMIT Latency ~ 45ms

» 872KB in flight if routers have wire-speed throughput

• Requirements for good performance: – Local area: minimize overhead/improve bandwidth– Wide area: keep pipeline full!

Lec 22.9

Sequence Numbers• Ordered Messages

– Several network services are best constructed by ordered messaging

» Ask remote machine to first do x, then do y, etc.– Unfortunately, underlying network is packet

based:» Packets are routed one at a time through the

network» Can take different paths or be delayed individually

– IP can reorder packets! P0,P1 might arrive as P1,P0

• Solution requires queuing at destination – Need to hold onto packets to undo misordering– Total degree of reordering impacts queue size

• Ordered messages on top of unordered ones:– Assign sequence numbers to packets

» 0,1,2,3,4…..» If packets arrive out of order, reorder before

delivering to user application» For instance, hold onto #3 until #2 arrives, etc.

– Sequence numbers are specific to particular connection

» Reordering among connections normally doesn’t matter

– If restart connection, need to make sure use different range of sequence numbers than previously…

Lec 22.10

Reliable Message Delivery: the Problem• All physical networks can garble and/or drop

packets– Physical media: packet not transmitted/received

» If transmit close to maximum rate, get more throughput – even if some packets get lost

» If transmit at lowest voltage such that error correction just starts correcting errors, get best power/bit

– Congestion: no place to put incoming packet» Point-to-point network: insufficient queue at

switch/router» Broadcast link: two host try to use same link» In any network: insufficient buffer space at

destination» Rate mismatch: what if sender send faster than

receiver can process?• Reliable Message Delivery on top of Unreliable

Packets– Need some way to make sure that packets actually

make it to receiver» Every packet received at least once» Every packet received at most once

– Can combine with ordering: every packet received by process at destination exactly once and in order

Lec 22.11

Using Acknowledgements

• How to ensure transmission of packets?– Detect garbling at receiver via checksum, discard

if bad– Receiver acknowledges (by sending “ack”) when

packet received properly at destination– Timeout at sender: if no ack, retransmit

• Some questions:– If the sender doesn’t get an ack, does that mean

the receiver didn’t get the original message?» No

– What if ack gets dropped? Or if message gets delayed?

» Sender doesn’t get ack, retransmits. Receiver gets message twice, acks each.

BAPacket

ack

BA

Packet

ack

PacketTimeout

Lec 22.12

BAPkt #0

Ack #0

Pkt #1

Ack #1Pkt #0

Ack #0

How to deal with message duplication• Solution: put sequence number in message to

identify re-transmitted packets– Receiver checks for duplicate #’s; Discard if

detected• Requirements:

– Sender keeps copy of unack’ed messages» Easy: only need to buffer messages

– Receiver tracks possible duplicate messages» Hard: when ok to forget about received message?

• Alternating-bit protocol:– Send one message at a time; don’t send

next message until ack received– Sender keeps last message; receiver

tracks sequence # of last message received• Pros: simple, small overhead• Con: Poor performance

– Wire can hold multiple messages; want tofill up at (wire latency throughput)

• Con: doesn’t work if network can delayor duplicate messages arbitrarily

Lec 22.13

BA

Better messaging: Window-based acknowledgements

N=5 Qu

eu

e

ack#0

ack#4

pkt#0

pkt#4

• Window based protocol (TCP):– Send up to N packets without ack

» Allows pipelining of packets» Window size (N) < queue at destination

– Each packet has sequence number» Receiver acknowledges each packet» Ack says “received all packets up

to sequence number X”/send more• Acks serve dual purpose:

– Reliability: Confirming packet received– Flow Control: Receiver ready for packet

» Remaining space in queue at receiver can be returned with ACK

• What if packet gets garbled/dropped? – Sender will timeout waiting for ack packet

» Resend missing packets Receiver gets packets out of order!

– Should receiver discard packets that arrive out of order?

» Simple, but poor performance– Alternative: Keep copy until sender fills in missing

pieces? » Reduces # of retransmits, but more complex

• What if ack gets garbled/dropped? – Timeout and resend just the un-acknowledged

packets

Lec 22.14

Transmission Control Protocol (TCP)

• Transmission Control Protocol (TCP)– TCP (IP Protocol 6) layered on top of IP– Reliable byte stream between two processes on

different machines over Internet (read, write, flush)

• TCP Details– Fragments byte stream into packets, hands

packets to IP» IP may also fragment by itself

– Uses window-based acknowledgement protocol (to minimize state at sender and receiver)

» “Window” reflects storage at receiver – sender shouldn’t overrun receiver’s buffer space

» Also, window should reflect speed/capacity of network – sender shouldn’t overload network

– Automatically retransmits lost packets– Adjusts rate of transmission to avoid congestion

» A “good citizen”

Router Router

Stream in: Stream out:

..zyxwvuts gfedcba

Lec 22.15

TCP Windows and Sequence Numbers

• Sender has three regions: – Sequence regions

» sent and ack’ed» Sent and not ack’ed» not yet sent

– Window (colored region) adjusted by sender• Receiver has three regions:

– Sequence regions» received and ack’ed (given to application)» received and buffered» not yet received (or discarded because out of

order)

Sequence Numbers

Sentnot acked

Sentacked

Not yetsent Sender

Not yetreceived

ReceivedGiven to app

ReceivedBuffered Receiver

Lec 22.16

Seq

:190

Size

:40

Window-Based Acknowledgements (TCP)

Seq:230 A:190/140

Seq:260 A:190/100

Seq:300 A:190/60

Seq:190 A:340/60

Seq:340 A:380/20

Seq:380 A:400/0

A:100/300

Seq:100 A:140/260

Seq:140 A:190/210

100

Seq

:100

Size

:40

140

Seq

:140

Size

:50

190

Seq

:230

Size

:30

230 260

Seq

:260

Size

:40

300

Seq

:300

Size

:40

340

Seq

:340

Size

:40

380

Seq

:380

Size

:20

400

Retransmit!

Lec 22.17

Selective Acknowledgement Option (SACK)

• Vanilla TCP Acknowledgement– Every message encodes Sequence number and

Ack– Can include data for forward stream and/or ack

for reverse stream• Selective Acknowledgement

– Acknowledgement information includes not just one number, but rather ranges of received packets

– Must be specially negotiated at beginning of TCP setup

» Not widely in use (although in Windows since Windows 98)

IP H

ead

er

(20 b

yte

s)

Seq

uen

ce N

um

ber

Ack N

um

ber

TCP Header

IP H

ead

er

(20 b

yte

s)

Seq

uen

ce N

um

ber

Ack N

um

ber

TCP Header

Lec 22.18

Congestion Avoidance• Congestion

– How long should timeout be for re-sending messages?

» Too longwastes time if message lost» Too shortretransmit even though ack will arrive

shortly– Stability problem: more congestion ack is

delayed unnecessary timeout more traffic more congestion

» Closely related to window size at sender: too big means putting too much data into network

• How does the sender’s window size get chosen?– Must be less than receiver’s advertised buffer

size– Try to match the rate of sending packets with the

rate that the slowest link can accommodate– Sender uses an adaptive algorithm to decide size

of N» Goal: fill network between sender and receiver» Basic technique: slowly increase size of window

until acknowledgements start being delayed/lost• TCP solution: “slow start” (start sending

slowly)– If no timeout, slowly increase window size

(throughput)– Timeout congestion, so cut window size in half– “Additive Increase, Multiplicative Decrease”

Lec 22.19

Sequence-Number Initialization• How do you choose an initial sequence number?

– When machine boots, ok to start with sequence #0?

» No: could send two messages with same sequence #!

» Receiver might end up discarding valid packets, or duplicate ack from original transmission might hide lost packet

– Also, if it is possible to predict sequence numbers, might be possible for attacker to hijack TCP connection

• Some ways of choosing an initial sequence number:

– Time to live: each packet has a deadline.» If not delivered in X seconds, then is dropped» Thus, can re-use sequence numbers if wait for all

packets in flight to be delivered or to expire– Epoch #: uniquely identifies which set of sequence

numbers are currently being used» Epoch # stored on disk, Put in every message» Epoch # incremented on crash and/or when run out

of sequence #– Pseudo-random increment to previous sequence

number» Used by several protocol implementations

Lec 22.20

Use of TCP: Sockets• Socket: an abstraction of a network I/O queue

– Embodies one side of a communication channel» Same interface regardless of location of other end» Could be local machine (called “UNIX socket”) or

remote machine (called “network socket”)– First introduced in 4.2 BSD UNIX: big innovation

at time» Now most operating systems provide some notion

of socket• Using Sockets for Client-Server (C/C++

interface):– On server: set up “server-socket”

» Create socket, Bind to protocol (TCP), local address, port

» Call listen(): tells server socket to accept incoming requests

» Perform multiple accept() calls on socket to accept incoming connection request

» Each successful accept() returns a new socket for a new connection; can pass this off to handler thread

– On client: » Create socket, Bind to protocol (TCP), remote

address, port» Perform connect() on socket to make connection» If connect() successful, have socket connected to

server

Lec 22.21

Socket Example (Java)server:

//Makes socket, binds addr/port, calls listen()ServerSocket sock = new ServerSocket(6013);while(true) {

Socket client = sock.accept();PrintWriter pout = new

PrintWriter(client.getOutputStream(),true);

pout.println(“Here is data sent to client!”);…

client.close();}

client:// Makes socket, binds addr/port, calls connect()Socket sock = new Socket(“169.229.60.38”,6013);

BufferedReader bin = new BufferedReader(

new InputStreamReader(sock.getInputStream));String line;while ((line = bin.readLine())!=null)

System.out.println(line);sock.close();

Lec 22.22

Distributed Applications• How do you actually program a distributed

application?– Need to synchronize multiple threads, running on

different machines » No shared memory, so cannot use test&set

– One Abstraction: send/receive messages» Already atomic: no receiver gets portion of a

message and two receivers cannot get same message

• Interface:– Mailbox (mbox): temporary holding area for

messages» Includes both destination location and queue

– Send(message,mbox)» Send message to remote mailbox identified by mbox

– Receive(buffer,mbox)» Wait until mbox has message, copy into buffer, and

return» If threads sleeping on this mbox, wake up one of

them

Network

Sen

d

Receiv

e

Lec 22.23

Using Messages: Send/Receive behavior

• When should send(message,mbox) return?– When receiver gets message? (i.e. ack received)– When message is safely buffered on destination?– Right away, if message is buffered on source

node?

• Actually two questions here:– When can the sender be sure that the receiver

actually received the message?– When can sender reuse the memory containing

message?

• Mailbox provides 1-way communication from T1T2

– T1bufferT2– Very similar to producer/consumer

» Send = V, Receive = P» However, can’t tell if sender/receiver is local or

not!

Lec 22.24

Messaging for Producer-Consumer Style

• Using send/receive for producer-consumer style:

Producer:int msg1[1000];while(1) {

prepare message; send(msg1,mbox);

}Consumer:

int buffer[1000];while(1) {

receive(buffer,mbox);process message;

}

• No need for producer/consumer to keep track of space in mailbox: handled by send/receive

– One of the roles of the window in TCP: window is size of buffer on far end

– Restricts sender to forward only what will fit in buffer

SendMessage

ReceiveMessage

Lec 22.25

Messaging for Request/Response communication

• What about two-way communication?– Request/Response

» Read a file stored on a remote machine» Request a web page from a remote web server

– Also called: client-server» Client requester, Server responder» Server provides “service” (file storage) to the client

• Example: File serviceClient: (requesting the file)

char response[1000];

send(“read rutabaga”, server_mbox);receive(response, client_mbox);

Consumer: (responding with the file)char command[1000], answer[1000];

receive(command, server_mbox);decode command;read file into answer;send(answer, client_mbox);

RequestFile

GetRespons

e

ReceiveRequestSend

Response

Lec 22.26

General’s Paradox• General’s paradox:

– Constraints of problem: » Two generals, on separate mountains» Can only communicate via messengers» Messengers can be captured

– Problem: need to coordinate attack» If they attack at different times, they all die» If they attack at same time, they win

– Named after Custer, who died at Little Big Horn because he arrived a couple of days too early

• Can messages over an unreliable network be used to guarantee two entities do something simultaneously?

– Remarkably, “no”, even if all messages get through

– No way to be sure last message gets through!

11 am ok?

So, 11 it is?Yes, 11 works

Yeah, but what it you

Don’t get this ack?

Lec 22.27

Two-Phase Commit• Since we can’t solve the General’s Paradox (i.e.

simultaneous action), let’s solve a related problem

– Distributed transaction: Two machines agree to do something, or not do it, atomically

• Two-Phase Commit protocol does this– Use a persistent, stable log on each machine to

keep track of whether commit has happened» If a machine crashes, when it wakes up it first

checks its log to recover state of world at time of crash

– Prepare Phase:» The global coordinator requests that all participants

will promise to commit or rollback the transaction» Participants record promise in log, then

acknowledge» If anyone votes to abort, coordinator writes “abort”

in its log and tells everyone to abort; each records “abort” in log

– Commit Phase:» After all participants respond that they are

prepared, then the coordinator writes “commit” to its log

» Then asks all nodes to commit; they respond with ack

» After receive acks, coordinator writes “got commit” to log

– Log can be used to complete this process such that all machines either commit or don’t commit

Lec 22.28

Two phase commit example• Simple Example: AATM machine, BThe Bank

– Phase 1:» A writes “Begin transaction” to log

AB: OK to transfer funds to me?» Not enough funds:

BA: transaction aborted; A writes “Abort” to log» Enough funds:

B: Write new account balance to loggBA: OK, I can commit

– Phase 2: A can decide for both whether they will commit

» A: write new account balance to log» Write “commit” to log» Send message to B that commit occurred; wait for

ack» Write “Got Commit” to log

• What if B crashes at beginning? – Wakes up, does nothing; A will timeout, abort and

retry• What if A crashes at beginning of phase 2?

– Wakes up, sees transaction in progress; sends “abort” to B

• What if B crashes at beginning of phase 2?– B comes back up, look at log; when A sends it

“Commit” message, it will say, oh, ok, commit

Lec 22.29

Distributed Decision Making Discussion• Two-Phase Commit: Blocking

– A Site can get stuck in a situation where it cannot continue until some other site (usually the coordinator) recovers.

– Example of how this could happen:» Participant site B writes a “prepared to commit”

record to its log, sends a “yes” vote to the coordintor (site A) and crashes

» Site A crashes» Site B wakes up, check its log, and realizes that it

has voted “yes” on the update. It sends a message to site A asking what happened. At this point, B cannot change its mind and decide to abort, because update may have committed

» B is blocked until A comes back– Blocking is problematic because a blocked site

must hold resources (locks on updated items, pagespinned in memory, etc) until it learns fate of update

• Alternative: There are alternatives such as “Three Phase Commit” which don’t have this blocking problem

Lec 22.30

Conclusion• Layering: building complex services from

simpler ones• Datagram: an independent, self-contained

network message whose arrival, arrival time, and content are not guaranteed

• Performance metrics– Overhead: CPU time to put packet on wire– Throughput: Maximum number of bytes per

second– Latency: time until first bit of packet arrives at

receiver• Arbitrary Sized messages:

– Fragment into multiple packets; reassemble at destination

• Ordered messages:– Use sequence numbers and reorder at destination

• Reliable messages:– Use Acknowledgements– Want a window larger than 1 in order to increase

throughput• TCP: Reliable byte stream between two

processes on different machines over Internet (read, write, flush)

• Two-phase commit: distributed decision making